1. Field of the Invention
The present invention relates to methods and apparatuses for implementing magnetic resonance procedures, including imaging and spectroscopy procedures, wherein the basic magnetic field is shimmed.
2. Description of the Prior Art
Magnetic resonance imaging and spectroscopy are patient examining modalities wherein magnetic resonance signals are caused to be generated in and emitted from an examination subject, and the emitted magnetic resonance signals are detected. Many known techniques are available for generating and detecting such signals. Imaging or spectroscopic information is then derived from these signals, also in a known manner.
In all magnetic resonance examinations, the patient is placed in a strong magnetic field that causes the nuclear spins in the examination subject to be aligned. Radio-frequency energy is then radiated into the patient which causes the nuclear spins to be displaced from the aligned orientation, so that the spins precess with a frequency that is dependent on the substance containing the precessing nuclei.
The strong magnetic field that initially aligns the nuclei (nuclear spins) is known as the B0 field, and is also called the basic magnetic field. This magnetic field is generally a static magnetic field, and must exhibit a high degree of homogeneity in the examination volume from which the magnetic resonance signals are acquired. Since the environment in which a particular magnetic resonance apparatus is installed, and the presence of the patient in the magnetic resonance apparatus, effect the homogeneity of the basic magnetic field, it is not possible to precisely and reproducibly set or fix the homogeneity of the basic magnetic field prior to installation of the magnetic resonance apparatus, and for many types of magnetic resonance examinations, the basic magnetic field must be adjusted immediately prior to the examination itself. The adjustment of the basic magnetic field is known as “shimming.”
A substantial number of clinical and research magnetic resonance scans are unusable due to patient motion that occurs during the scanning. In particular, magnetic resonance spectroscopy studies in the abdomen are very difficult to implement in practice, and require very good shimming of the basic magnetic field. Various techniques have been proposed to address this problem, including ECG triggering in order to gate the shimming at the same cardiac phase in each image acquisition (scan), as well as the breath-holding technique. The latter technique, however, cannot be used for patients who are unable to hold their breath, since conventional shim sequences usually employ 3D data acquisitions, which require a relatively long scan time. Very special need for techniques that allow the patient to freely breath during the shimming measurements that are made with the examination subject in the magnetic resonance apparatus.
Artifacts due to body motion such as respiratory motion are a problem in the field of magnetic resonance imaging, separate from the aforementioned shimming problem. Techniques are known to acquire magnetic resonance data, while allowing free breathing on the part of the patient, and the image data are then corrected according to motion compensation algorithms or other techniques. One such motion compensation technique is the 2D perspective acquisition correction (PACE) technique that is commercially available from Siemens Healthcare in a software package for operating the Magnetom Avanto. The PACE technique represents a consolidated and extremely rapid technique using embedded navigators to estimate and correct the motion of the diaphragm in real-time, without the need for additional hardware. This technique has proven successful as long as the organ displacement is modest. In this technique, the gradients are adjusted rapidly for rotations during scanning, and the RF slab select and phase errors due to the translations are also corrected in real-time. The resulting k-space data are therefore already corrected for motion, and the image can be reconstructed in a conventional manner, without the need for additional post-processing, to yield a motion-corrected image immediately at the display console. Frequency drift and first-order shim errors are also corrected in real-time.